Heating unit



FIG. 1 is a perspective view of a heating unit showing my new design;

FIG. 2 is a top view thereof;

FIG. 3 is a front view thereof, the back, left side, and right side views being identical thereto; and,

FIG. 4 is a side view thereof rotated about its central axis approximately 45 degrees from the position shown in FIG. 3.

The appearance of the bottom side forms no part of the claimed design. The broken lines form no part of the claimed design. 

The ornamental design for a heating unit, as shown and described. 